The present invention relates to exploration seismic reflection surveying and more particularly, relates to the processing of exploration seismic reflection data to enhance information in seismic signals reflected from contrasts in elastic constants, velocities, and/or densities in the subsurface of the earth.
The methods of the present invention which are described herein are generally discussed in terms of compressional wave (PP) seismic data, acquisition, and processing, which is the most common form of seismic data used in exploration seismology. However, it should be understood that these methods are equally applicable to shear wave seismic data and to converted wave seismic data.
Conventional land or marine seismic acquisition techniques involve the use of an appropriate source to generate seismic energy and a set of receivers, spread out along or near the surface of the earth on land, or at or near the water surface or water bottom in a water covered area, to detect any reflected seismic signals due to seismic energy striking subsurface geologic boundaries. These signals are recorded as a function of time and subsequent processing of these time varying signals, i.e. seismic "traces" or seismic data, is designed to reconstruct an appropriate image of the geologic boundaries of the subsurface and to obtain information about the subsurface materials. In simplistic terms, this conventional process has a seismic wave, from a source of seismic energy, traveling down into the earth, reflecting from a particular geologic interface (i.e., a contrast in elastic constants, velocities, and/or densities), and returning to the surface, where it may be detected by an appropriate detector.
As noted in the above-referenced related application, seismic data is processed to obtain information about the subsurface over which seismic data has been acquired. More particularly, one conventional processing technique is hyperbolic normal moveout (NMO) velocity analysis and subsequent "stacking" of the NMO corrected data. However, such conventional NMO techniques ignore the offset dependence of the seismic data. The related application provides methods for more accurately estimating moveout velocities while obtaining consistent estimates of the offset dependence and the zero offset information of the seismic data.
The estimate of the offset dependence of the amplitude (as described in the related application) has been somewhat limited by the coupling of the offset dependence and moveout velocity. Further, the conventional process of hyperbolic normal moveout correction distorts the amplitude and frequency of the seismic data, such an effect is commonly referred to as "stretch". In practice with actual field data, the measured offset dependence is frequently contaminated by relatively small moveout velocity errors, as well as other errors related to the process of moveout correction.
Thus, a method for determining the effect of moveout velocity errors and other errors (such as stretch) on the measurement of such offset dependence for the case of small offsets is needed. Such a method can simultaneously determine a more accurate measure of amplitude variation with offset and normal incident reflection amplitude, as well as moveout velocity. However, it should be noted that given the correct moveout velocity and in the absence of other known effects, the techniques and methods in the related copending application provide the correct offset dependence. The terms "amplitude variation with offset", "variation with offset", "offset dependence", and "offset dependence of amplitude" are used interchangeably herein.
These and other limitations and disadvantages of the prior art are overcome by the present invention, however, and an improved method is provided for determining from seismic data, normal moveout velocities, and associated dynamic corrections that preserve amplitude versus offset information to provide better estimates of effective velocities, the normal incidence reflection amplitudes, and the offset dependence of the reflection amplitudes.